GRBs as reionization probes

戸谷友則（TOTANI, Tomonori）Department of Astronomy, Univ. of Tokyo

GRB Workshop 2015RIKEN, 2015 Aug. 31 - Sep. 2

Talk Plan✦ concentrate on “Lyα damping wing fitting analysis” to get constraints on IGM neutral fraction

✦ review on GRBs as a reionization probe: the status before GRB 130606A

✦ On the results from GRB 130606A @ z=5.9✦ the best opportunity ever for reionization study by GRBs✦ controversy between Gemini/Subaru/VLT? what’s the origin?

✦ On the effect of Lyα cross section formulae (as a function of wavelength) adopted✦ need to be careful in the “high precision GRB cosmology era”

Cosmic Reionization ✦ The Universe (hydrogen) became neutral at z~1100✦ the cosmic recombination✦ observed as CMB

✦ Hydrogen in IGM today is highly ionized ✦ the Gunn-Peterson Test

✦ The universe must have been reionized at around z~10✦ most likely by UV photons by first stars

✦ when? how? important benchmark to understand galaxy formation

Djorgovski+

The Gunn-Peterson Test

✦ Lyα absorption features of QSOs indicating that IGM neutral fraction rapidly increasing to z ~ 6✦ close to reionization?

✦ but saturated GP troughs only gives a lower limit of nHI/nH > 10-3

White+’03

Fan+’05

“GP troughs”

Observational Constraints on Reionization History

✦ Fan+ ’06

A Next Step: Using Lyα Red Damping Wing✦ measurement of fHI=nHI/nH , rather than lower limit, is possible if damping wing feature by neutral IGM is detected!

✦ GRBs especially powerful:✦ simple power-law unlike quasars✦ no proximity effect✦ more normal regions than quasars

✦ Obstacles: ✦ low event rate of high-z GRBs✦ contamination by HI in host galaxies

✦ GRB 050904: the first meaningful constraint ✦ 95% C.L. upper limit fHI < 0.6 (TT+’06) GRB 050904@z=6.3, TT+ ‘06

GP trough → fHI > 10-3

damping wing → measure fHI

Observational Constraints on Reionization History

✦ Chornock+ ’14

Planck’13: zre = 11.4+4.0-2.8

The best opportunity ever: GRB 130606A

✦ exceptionally bright afterglow

✦ ultra-high S/N spectra taken by Gemini, GTC, Magellan, Subaru, VLT, ...

✦ host HI at most log(NHI)< 19.8, good for IGM study!✦ c.f. 21.6 for GRB 050904

Chornock+’13

Gemini vs. Subaru vs. VLT✦ Chornock et al. 2013 (Gemini, ApJ, 774, 26)

✦ no evidence for IGM HI by damping wing analysis✦ fHI < 0.11 (2σ）✦ spectral index β=-1.99 (fν∝νβ), very different from β～-1 found by more recent studies

✦ Totani et al. 2014 (Subaru, PASJ, 66, 63)✦ ~3σ preference for IGM HI, with

✦ fHI ~ 0.09 if zIGM, u = zGRB = 5.913 (β=-0.93)✦ fHI ~ 0.4 if zIGM, u = 5.83 < 5.913 (β=-0.74) ← now disfavored from VLT measurement of β

✦ Hartoog et al. 2014 (VLT, arXiv:1409.4804)✦ β=-1.02 from optical-NIR spectrum ✦ no evidence for IGM HI, fHI < 0.03 (3σ)

Damping Wing Analysis for Subaru Data✦ Subaru/FOCAS spectrum in 10.4-13.2 hr after the burst✦ S/N=100 per pixel (0.74A)!✦ 8400-8900 A which is the most sensitive to IGM HI signature✦ avoid strong absorption

TT+’14

Fitting Residuals ✦ power-law + host HI only

✦ free parameters: power-law index, NHI, σv

✦ showing curved systematic residual✦ amplitude ~ 0.6% of continuum flux

✦ diffuse IGM HI can reduce the residual by about 3 sigma statistics ✦ fHI ~ 0.1, if IGM extending to zu=zGRB=5.913

TT+’14

DW from various components✦ wavelength close to Lyα center is dominated by HI in the host galaxy

✦ IGM HI becomes relatively important at wavelength far from Lyα

✦ wavelength range choice is a crucial issue in the damping wing analysis for reionzation!

TT+’14

Very subtle! systematics?

✦ various sources of systematics examined, but unlikely to explain the 0.6% curvature in the narrow range of 8400-8900 A✦ spectrum reduction, calibration

✦ calibration accuracy is < 0.2%✦ no known systematics can explain the observed curvature

✦ extinction at host ✦ extinction does not explain the strong curvature in the short wavelength range

✦ DLAs on the sightline ✦ disfavored from Lyβ and metal absorption

TT+’14

✦ To reveal this, the Subaru and VLT spectra have been exchanged by the two teams✦ I thank the VLT team for kindly agreeing with this exchange

✦ VLT spectrum averaged on the Subaru spectrum grids✦ VLT has a better spectral resolution✦ S/N similar per wavelength

✦ no systematic trend on > 100 Å scale

✦ how about adopting the same Subaru analysis code on the VLT spectrum?

what’s the origin of Subaru/VLT controversy?

TT+’15

Result of TT’s-code on VLT spectrum. 1

✦ βfixed at -1.02 as measured by VLT✦ IGM HI extends to zGRB,u = zGRB = 5.913

✦ The original Subaru result (~3σ preference for IGM HI) confirmed using VLT spectrum

Result of Subaru-code on VLT spectrum. 2

✦ the same trend for the fit residuals by no IGM HI model

Subaru data VLT data

What’s the origin of discrepancy?✦ wavelength ranges used are very different for Subaru and VLT papers

✦ 8406-8462 Å by VLT✦ 8426-8900 Å by Subaru (<8426Å avoided because of strong dependence on host HI velocity distribution)

✦ when the TT’s-code adopted on the VLT spectrum, I confirmed the VLT paper result (no evidence for host HI)

✦ the VLT-paper range is highly sensitive to velocity distribution of HI in the host✦ σv = 61.8±3.3 km/s by our fit result✦ systematics about unknown realistic velocity distribution is a worry

range adopted by VLT paperwhite regions used in Subaru paper

On the Lyα cross section formulae✦ classical Rayleigh scattering

✦ Lorentzian

✦ Peebles’ two-level approximation

✦ second order perturbation theory for fully quantum mechanical scattering (Bach+’14)

effect on HI opacity by Lyα cross section formulae

✦ ~10% difference in cross section / HI opacity

✦ The Peebles’ formulae often used shows the largest deviation from BL (Bach-Lee) formula

✦ How much is the effect on the damping wing fitting results?✦ perhaps the evidence for IGM HI reported by TT+’14 just an artifact by using inaccurate cross section formula?

GRB 130606A case

Fitting results dependence on cross section formulae✦ on the Subaru data of the GRB 130606A spectrum✦ with the fitting method of TT+’14, only changing Lyα cross section formula

✦ preference to IGM HI by ~3-4σ unchanged

Conclusions (1/2)✦ GRB 130606A gives the best ever opportunity to probe reionization by GRBs

✦ ~3σ evidence for IGM HI is found by the damping wing analysis of Subaru spectrum✦ fHI ~ 0.1 if zIGM,u = zGRB = 5.913 ✦ robust against known systematics (spectrum, extinction, intervening DLA)✦ the first evidence for intervening HI to GRB sightlines✦ suggesting that the reionization not yet complete at z~6, but needs more sightlines to examine inhomogeneity

✦ discrepant result from VLT (Haartoog et al. 2014)?✦ data consistent with each other, and the same result confirmed when the same analysis is done on the two different spectra✦ high precision damping wing analysis indeed possible!

✦ discrepancy comes from different wavelength ranges✦ need to be careful for systematics in analysis methods!✦ systematics about host HI velocity distribution seems serious when using range close to Lyα resonance

Conclusions (2/2)✦ Now we are in the era of “GRBs as a high precision reionzation probe”✦ sensitive to fHI ~ 0.1 at z~6!✦ systematics must be carefully treated

✦ Choice of Lyα cross section formulae is important in a high-precision analysis such as GRB 130606A✦ but preference to IGM HI reported by TT+’14 unchanged

✦ current limitation of GRBs as a reionization probe:✦ low event rate of sufficiently bright GRBs at z>~6✦ this situation will be improved by 30m-class telescopes✦ future GRB missions in synergy with 30m-class telescopes will be crucial

✦ good data for many GRBs would reveal not only the mean but also inhomogeneity of reionization history